1. Field of the Invention
The present invention relates to a battery charger and has a particularly advantageous application in the field of batteries for mobile radio terminals.
2. Description of the Prior Art
FIG. 1 shows a conventional mobile radio terminal charger device as used in the prior art. The charger device 11 comprises a charger 8 with two input terminals connected to the AC mains 12. Two output terminals of the charger 8 are connected to the battery 6 via a charging switch 13. The charger device 11 also includes a charging control integrated circuit 14 connected to the battery 6 and connected to a sensor 10 for sensing the temperature of the battery 6 by a connection 16. The charging control circuit 14 also receives information corresponding to the battery voltage via the connection 15. The control circuit 14 controls opening and closing of the charging switch 13 to start and stop charging of the battery 6. The control circuit 14 is also connected to the charger 8 to detect the presence of the charger 8. The presence of the charger is detected by measuring the output voltage of the charger via the connection 17. The control circuit 14, the detector 10 and the switch 13 are inside the mobile radio terminal. The charger 8 has a two-fold function of rectification, filtering and regulation to obtain from the AC mains 12 the appropriate current and voltage for charging the battery 6.
On the basis of information obtained via the connections 15 and 16, the control circuit 14 starts and stops charging of the battery 6 by closing or opening the charging switch 13. For a lithium ion battery the control circuit 14 uses information on the battery voltage whereas for an NiMH battery it uses information on the battery temperature. In all cases, the circuit 14 receives information from the charger 8 corresponding to the charger output voltage and enabling it to detect that the charger is present. This voltage may be relatively high when the switch 13 is open since the battery is no longer charging; it can reach values from around 10 volts to around 20 volts. Some components used in integrated circuits, such as CMOS components, cannot withstand such voltages.
One solution is to use in the charging control circuit BICMOS bipolar components able to withstand higher voltages.
However, this solution gives rise to problems since BICMOS components are much more costly than CMOS components. Moreover, the CMOS technology is much more widespread and standardized than the BICMOS technology.
Furthermore, it is increasingly difficult to distinguish between charger designs, in particular pirated designs. It is important to be able to determine the authenticity of a charger, in particular for reasons of safety and utilization standards.
One solution consists in measuring the off-load voltage of a charger and considering that it is not pirated if the voltage is within a predetermined fixed range.
However, this leads a relatively wide margin of error in that all chargers have fairly similar off-load voltages.
The present invention seeks to provide a battery charger allowing, firstly, the use in the charging control circuit of components such as CMOS components able to withstand only low voltages and, secondly, effective determination of its authenticity.